Paper of improved wet strength



paper.

Patented June 8, 1954 UNITED STAT PAPER F IMPROVED WET STRENGTH New YorkNo Drawing. Application J anuary'26, 1950, Serial No. 140,746

4 Claims.

The invention relates to paper which contains a synthetic resin thatimparts wet strength, and which is superior to the synthetic resinimpregnated paper heretofore known.

When used for imparting wet strength to paper, a synthetic resin isusually incorporated at the wet end of the paper making process, forexample, in the beater or at the head box. A synthetic resin that isincorporated at the wet end of the paper making process must be capableof dilution without recipitation of the resin, and must have an afiinityfor the paper fibers so that a reasonably large proportion of the resindeposits on the paper fibers and so that an unreasonably largeproportion of the resin is not lost in the waste water.

The principal object of the invention is to provide paper comprising asynthetic resin that has the special properties necessary to permit itto be incorporated at the wet end of the paper making process and alsoimparts substantially greater wet strength per unit of cost than theresins heretofore used for the wet-strengthening of More specificobjects and advantages are apparent from the description, whichillustrates and discloses but is not intended to limit the scope of theinvention.

The present invention is based upon the discovery that paper has wetstrength superior to that heretofore known if it contains a product ofthe reaction of formaldehyde, a substituted aliphatic carboxylic acidwhose molecule has not more than eight carbon atoms and has not morethan four carbon atoms per carboxy group, and a substance of the classconsisting of urea and melamine, the monovalent substituents in saidreaction product that are derived from said reactants consisting of NH2,OH and COOH groups.

The formaldehyde used in producing paper embodying the invention may bewholly or partially in the form of one of its polymers such asparaformaldehyde, but preferably is in the form of an aqueous solution(as hereinafter described).

The term resin former is used herein to refer to a substance of theclass consisting of urea and melamine. The resin former may consist ofeither urea or melamine or a combination of both in any desiredproportions. Urea is the preferred resin former, especially since it isless expensive to use than melamine.

The substituted carboxylic acid used in the practice of the presentinvention may be any sub- :stance whose molecule consists of asubstituted straight or branched hydrocarbon chain having not more thaneight carbon atoms, in which the substituents include at least onecarboxy group and at least one hydroxy or amino group, each attached toa primary, secondary, or tertiary carbon atom, said molecule having nosubstituents other than carboxy, hydroxy or amino groups and having notmore than four carbon atoms per carboxy group, i. e., having from one tothree additional carbon atoms per carboxy group. Preferably, thesubstituents are attached to primary or secondary carbon atoms. Suchsubstances include: glycolic acid, lactic acid, glycine, alanine,alpha-amino butyric acid, alpha-amino iscbutyric acid, aspartic acid,glutamic acid, bydroxyglutaric acid, hydroxyglutamic acid,hydroxybutyric acid, malic acid, serine, tartaric acid, and threonine.For the sake of brevity the term substituted carboxylic acid will beused hereinafter to designate such a substance.

The paper pulp which may be used in the practice of the invention may beof any type, such as bleached or unbleached K ra-ft, sulfite, or groundwood pulp. I

In the practice of the invention when the resin former is urea, notinore than about .2 mols of formaldehyde should be used per mol of urea,and it is preferable to use not more than about 2.15 mols per mol ofurea. Not less than about 1.8 mols of formaldehyde should be used permol of urea and it is preferable to use not less than about 1.9 mols permol of urea. It is most desirable to use about 2.12 mols of formaldehydeper mol of urea. When the resin former is melamine at least 4 mols offormaldehyde should be used per mol of melamine, and not more than about'7 mols of formaldehyde per mol of melamine should be used. It ispreferable to use at least about 6 mols of formaldehyde per mol ofmelamine and most desirable to use about 6% mols of formaldehyde per molof melamine.

In the practice of the invention not less than .05 mol of substitutedcarboxylic acid should be used per mol of urea. It is preferable thatthe amount of substituted carboxylic acid used be not less than about.15 mol per mol of urea. It is preferable to use not more than about .25mol of substituted carboxylic acid per mol of urea, and most desirableto use about .2 mol of substituted carboxylic acid per mol of urea.Increasing the amount of substituted carboxylic acid'to as much as .5mol per mol of urea tends to decrease the degree of wet strengthimparted to paper embodying the invention.

It is preferred that wet-strengthened paper embodying the inventioncomprise a product of the reaction of formaldehyde, a resin former and asubstituted carboxylic acid in which carboxy radicals are neutralizedwith an alkali metal base (as hereinafter explained).

The term alkali metal base is used herein to mean any alkali metalcompound that gives an alkaline solution, the preferred alkali metalsbeing sodium and potassium. Such alkali metal bases include sodiumhydroxide, potassium hydroxide, sodium alcoholates such as sodiummethoxide, sodium ethoxide, and sodium beta-methoxy ethoxide (obtainedsimply by dissolving metallic sodium in methanol, ethanol orbeta-methoxy ethanol, respectively), potassium carbonate, sodiumtetraborate, potassium tetraborate, and sodium bicarbonate. Thepreferred alkali metal bases are sodium hydroxide and potassiumhydroxide. Sodium hydroxide is more desirable from the standpoint ofeconomy.

It is desirable that wet-strengthened paper embodying the inventioncomprise a product of the reaction of formaldehyde, a resin former, anda substituted carboxylic acid which has two carbon atoms (for example,glycolic acid, glycine, or serine), in which carboxy radicals areneutralized with an alkali metal base. In the practice of the inventionthe preferred substituted carboxylic acid having two carbon atoms isglycolic acid.

Thus the most desirable finished article embodying the inventionconsists of wet-strengthened paper impregnated with a product of thereaction of formaldehyde, urea, glycolic acid and sodium hydroxide.

Wet-strengthened paper is extremely useful in all paper products whichmay come in contact with water or moisture during use, for exampletoweling, food product wrappers, bag and wrapping paper, and map andblue print paper.

A synthetic resin for imparting wet strength to paper is desirablyincorporated at the wet end of the paper making process before the paperis made. This is known as wet end addition, the term referring not onlyto addition in the beater but also to addition in the machine chest,head box, fan pump or any other desired point at the wet end of thepaper making process. This more convenient and less expensive method ofapplying the resin to the paper results in a more porous paper which isnot coated with a sealing. Since in the production of Wet strengthpaper, the mixture at the point of resin addition ordinarily comprises avery dilute suspension of pulp in water (less than two percent) and asynthetic resin used for imparting wet strength is usually present inthis suspension in a concentration of about one to two per cent of thepulp concentration, the resin must be capable of dilution withoutprecipitation. Such a resin should be a thermosetting composition sothat it can b added in its soluble stage to disperse and dissolvethroughout the paper pulp suspension at the wet end of the paper makingprocess before the paper is made, and then can be converted to athermoset resin on the paper fibers by heating during drying or agingduring storage.

Although theoretically ordinary urea-formaldehyde (andmelamine-formaldehyde) resins are soluble in the concentration rangerequired during wet end addition, in actual practice these thermosettingresins form curds when incorporated at the wet end under the slightlyacid conditions used, before they have had time to be dispersed anddissolved in the water. The curds cannot be readily dissolved, and stickto the apparatus so that much of the resin is deposited on the walls ofthe apparatus or lost in the Waste water. (The formation of curdsnecessitates frequent cleaning of the apparatus and leads to seriousdimculties, because under the acid conditions used the curds, afterdepositing on the equipment are converted to the insoluble state.) Thesmall amount of resin that does cling to the paper fibers does not coatthem uniformly. Such a resin makes the paper non-absorbent and isundesirable for use in imparting wet strength to paper.

Since urea-formaldehyde (and melamineformaldehyde) resins when used aspaper treating resins to impart a high degree of wet strength must besufficiently water soluble to dissolve so rapidly that curds do notform, it is necessary to use a modifying agent which gives a high degreeof water solubility to such resins. It has been found that this isaccomplished by the presence of an electrolytic group attached to theresin molecule, which gives it an electrical charge when in solution.The substituted carboxylic acid, or a salt thereof, used in the practiceof the present invention becomes part of the resin through its amine orhydroxy substituent. The ionization in solution of the substitutedcarboxylic acid which is thus a part of the resin molecule imparts anegative electric charge to the molecule. Such a soluble anionic resinimparts wet strength to to paper when it is converted to a thermosetresin on the paper fibers by heating during drying or aging duringstorage. It is believed that the presence of the solubilizing groups inthe resin molecules on the fiber also imparts greater absorbency to thepaper since the soluble resin is less water repellent and hencefacilitates passage of water through the thin layer of resin on thefiber. The known sodium :bisulfite modified resins do not have as goodwater solubility and do not impart as high a degree of wet strength topaper per pound of resin used as the synthetic resins used in thepractice of the present invention (as is hereinafter demonstrated).

Sodium bisulfite may be used along with the substituted carboxylic acidin the practice of the invention. Such a modifying agent should not bepresent in a concentration greater than about 30 per cent of thesubstituted carboxylic acid, since its use in larger amounts gives apaper having lower wet strength than the paper produced by using onlythe substituted carboxylic acid. (The term per cent as used herein torefer to quantities of material means per cent by weight unlessotherwise qualified.)

The reaction mixture in the preparation of a resin for use in thepresent invention usually contains formaldehyde in the form of anordinary commercial formaldehyde solution. Such a solution is in itselfhighly acid so that the pH of the reaction mixture containing also thesubstituted carboxylic acid is quite low. Since the reaction offormaldehyde and a resin former tends to go too rapidly in a stronglyacid solution so that the product coagulates, and since the reactiondoes not proceed satisfactorily if the solution is strongly alkaline, itis desirable that the pH of the reaction mixture be adjusted within therange 5.2 to 5.6. Varying the reaction pH between 5.2 and 5.6 andvarying the reaction temperature has no substantial effect in general onthe properties of resins reacted to the same degree of condensation.Since, however, undesirable by-products form within this pH range attemperatures below around degrees C. it is desirable to lower the pH towithin the range from 5.2 to 5.6 only after the temperature of thereaction mixture reaches about 95 degrees C., to avoid turbidity in thefinal product. (Although formation of an insoluble by-product has noeffect on the wet-strengthened paper made from filtered resin, thepresence of a precipitate makes colorimetric pH control during the resinpreparation very difficult.) It is, therefore, most desirable to raisethe initial pH with a base within a range of about 6.5 to 7.0 and tomaintain the pH of the mixture in approximately this range until thetemperature is about 95 degrees C.

It is preferable to use a strong base such an alkali metal base (ashereinbefore defined). Such a base will also neutralize part or all ofthe substituted carboxylic acid. At the time of reaction it is preferredthat from 20 to' 100 mol per cent of the substituted carboxylic acidtaking part in the reaction be in the form of a metal salt that is watersoluble and does not interfere in the reaction. The product of such areaction may be used in the production of wet-strengthened paperembodying the invention because the salt is large- 1y ionized in theaqueous reaction medium, and some of the anions produced by suchionization combine with hydrogen ions to form molecules of thecarboxylic acid so that some of the molecules taking part in thereaction are molecules of the free carboxylic acid. If the water presentduring the reaction is removed by drying the reaction product, and ifthe cations from the metal base are present in an amount sufficient tocombine with all of the carboxy groups in the molecules of the reactionproduct, the ionization which tool; place when the alkali metal saltdissolved in the aqueous reaction medium is reversed during the dryingof the reaction product and all the carboxy groups in the dried reactionproduct are neutralized by the metal base.

In the preparation of a resin for use in the practice of the presentinvention the reaction medium usually consists only of water, althoughit may contain small amounts of a solvent such as methyl alcohol orethyl alcohol. Usually the water is that present in the commercialaqueous formaldehyde solution ordinarily employed. The components may bemixed in any desired order. It is usually desirable to neutralize thecarboxylic acid first and then to add the formaldehyde solution, ureaand sufiicient base to adjust the pH of the mixture to 6.5-7.0. Themixture is then either heated for one hour at 60 degrees C. or allowedto stand overnight. However, resins imparting equally good wetstrengthening properties may be obtained without this much preliminaryreaction. It is necessary merely to allow enough time for the componentsto dissolve before reacting the mixture.

It is believed that during this preliminary reaction stage one moleculeof formaldehyde combines with one molecule of water to form methyleneglycol, and it is believed to be the methylene glycol that actuallytakes part in the reaction with a substance such as urea, so that amethylol group is formed by the condensation of a methylene glycolmolecule with an Nl-lz group in the urea molecule; one molecule of waterbeing eliminated in such condensation:

HCHO H2O canon OH H 0 on H o Thus in a molecule formed by the reactionof formaldehyde and urea the only monovalent substituents are hydroxygroups and amino groups.

Similarly, the only monovalent substituents in the molecules of thesubstittued carboxylic acids that are used in the practice of theinvention consist of hydroxy or amino groups and carboxy groups. It isbelieved that a hydroxy or amino group in the substituted carboxylicacid condenses with a hydroxy group in the resin-molecule.

Another procedure comprises preliminarily reacting the neutralizedsubstituted carboxylic acid with the formaldehyde solution for about twohours, prior to the addition of the resin former and sufiicient base toadjust the pH to the proper range. This procedure is essential whenmelamine is used as the resin former in order to obtain a resin whichwill develop wet strength since melamine reacts more rapidly than ureaand time must be allowed for the substituted carboxylic acid to reactwith the formaldehyde.

Still another procedure for preparing a synthetic resin for use in thepresent invention comprises adjusting the mixture of urea andformaldehyde solution to within the proper pH range and reacting forabout 45 minutes prior to the addition of the neutralized substitutedcarboxylic acid.

The degree of water solubility of resins used in the practice of thepresent invention is dependent upon the number of solubilizing groups"which become part of the resin molecule. Since increasing the amount ofsubstituted carboxylic acid beyond the desirable limits hereinbeforedescribed tends to decrease the degree of wet strength imparted topaper, it is not desirable to improve the water solubility by soincreasing the amount of substituted carboxylic acid.

The synthetic resins ordinarily used in the present invention to preparepaper superior to the synthetic resin-impregnated paper heretofore knownare reacted in the molar proportions hereinbefore described in anaqueous reaction medium derived from a commercial formaldehyde solution.An ordinary commercial formaldehyde solution usually consists of 37 percent formaldehyde and 67 per cent water. The only other source of waterin the reaction mixture is that present, if any, in the substitutedcarboxylic acid, which may contain about 30 per cent of water. Furtherdilution during the reaction is undesirable, since it results in resinsof decreased stability. It has been discovered that less dilution duringthe reaction, i. e., reacting at a higher solids concentration than isachieved using an ordinary commercial formeldehyde solution, results inresins having a greater degree of water solubility, since there is thena tendency for more of the solubilizing groups to become part of theresin molecule. Such resins impart higher wet strength to paper and,when diluted after the reaction as hereinafter described, have increasedstability. The solids concentration of the reaction mixture may beincreased suiilciently to yield resins having improved water solubilityby using the neutralized substituted carboxylic acid. in the form of itscrystalline salt. For example, use of crystaline sodium glycolateresults in a more soluble resin than is obtained with an equivalentamount of a solution of '70 per cent glycolic acid neutralized withflake caustic. Even greater improvement in the water solubility ofresins used in the practice of the invention is obtained if theconcentration of solids in the resin reaction mixture is increased byusing a formaldehyde solution containing a greater concentration offormaldehyde than 3'7 per cent. It is desirable to use an aqueoussolution containing a concentration of 45 to 50 per cent offormaldehyde, since such use of a concentrated formaldehyde solutiongives substantially improved properties economically and efilciently. Ifdesired sufilcient paraformaldehyde may be dissolved in a 37 per centaqueous formaldehyde solution to raise the concentration of formaldehydeto 45 or 50 per cent.

In general, more highly condensed resins impart better wet-strength topaper. However, the increase in wet strength imparted by a given resinmay be inappreciable beyond a certain degree of condensation(viscosity), and since increased condensation tends to decrease both thestability and the water solubility of the resin, the reaction should beterminated when that viscosity has been reached. The viscosity of resinsreacted to the same degree of condensation will, of course, difier inaccordance with the solids concentration of the reaction mixtures.Resins used in the practice of the present invention in which theproportions of resin reactants are within the limits hereinbefore givenand in which the concentration of formaldehyde in aqueous solution isapproximately (a) 3'7 per cent, (1)) 50 per cent or (c) 45 per cent arereacted to a desirable degree of condensation as follows:

(a) The pH of the reaction mixture is initially adjusted to 6.5-7.0, themixture is heated to a temperature of about 95 degrees C. beforelowering the pH to 5.2-5.6, and the reaction is continued at thistemperature until the viscosity of the resin solution is about 2'?seconds (measured by Ford cup, inch opening, at 25 degrees C.). Thereaction usually requires about eight to nine hours when the pH is about5.6, or from two to six hours when the pH is within the range 5.3 to5.45. The mixture is then cooled to 60 degrees C. and held at thistemperature until the viscosity of the solution is 32 to 34 seconds(Ford cup), the pH being adjusted to 5.6 at the beginning of thisperiod. This latter reaction stage usually requires from one to threehours.

(b) The pl-l of the reaction mixture is initially adjusted to 6.57.0,and the mixture is heated to a temperature of 95 degrees 0., twosubsequent pH adjustments being made, one to 7 .5 at 50 de grees C. andthe other to 7.2 at 75 degrees C'. to avoid precipitate formation. It isdesirable that the temperature rise from 65 degrees C. to 95 degrees C.in less than 45 minutes. The mixture is held at 95 degrees C. for fiveminutes before lowering the pH to about 5.6, and the solution is held atthis temperature and pH until the viscosity of the resin solution is J(measured by Gardner-Holdt Bubble Viscometer standard method, the Fordcup viscosity method becoming increasingly inaccurate at the much higherviscosity encountered using the more concentrated reaction mixture). Themixture is then rapidly cooled to 60 degrees C. (about 10 minutescooling time) and held at this temperature until the viscosity of thesolution is VW (Gardner- I-Ioldt) (c) The resin is prepared by theprocedure described in (5) except that the viscosity of the solutionafter the 95 degree stage should be H-- I and the final viscosity afterthe 60 degree stage should be U-V (Gardner-Holdt).

Using a two-stage reaction (that is, heating first at 95 degrees C. to acertain viscosity and then reaching the final viscosity by reacting at60 degrees C.) ordinarily makes the reaction more controllable and givesmore reproducible results than a one-stage reaction carried to the samefinal viscosity at degrees C., although there is no essential differencein the properties of the final product. Usually, the total reaction timemust be at least three to four hours in order to allow sufiicient timefor the viscosity measurements and the temperature and pH adjustmentsthat are necessary for safe control.

The wet strength of paper embodying the invention is increased when thepaper is treated With a resin which has been aged at room temperature oreven lower temperatures for as long a period of time as the resinremains stable. It is desirable to neutralize the liquid resin with abase such as sodium hydroxide to a pH of at least 7.0 and most desirableto adjust the pH to the range 7.8 to 8.0, for greater stability of theresin. The pH of the resin solution decreases on standing, the decreasebeing more rapid at higher temperatures. The decreasing pH of the resinsolution is probably at least partially due to the formation of formicacid through air oxidation (and Cannizzaro type reaction) of the freeformaldehyde usually present. Thereiore, a buffer such as sodiumbicarbonate or borax (0.5 per cent based on the weight of solids in theresin) or sodium borate (0.2 to 0.3 per cent based on the weight ofsolids in the resin) is usually added to retard the rate of pH drop.Resins prepared using a more concentrated reaction mixture are mostdesirably adjusted to a pH of 8.0 and buffered with 2 per cent (based onthe weight of solids in the resin) of a buffer mixture containing 65 percent boric acid and 35 per cent borax.

Resins reacted according to the procedure described in (a) ordinarilycontain about 48 to 50 per cent solids, and those reacted according toprocedures (19) and (c) ordinarily contain about 51 to 55 per centsolids. The stability of resins used in the practice of the inventionmay be further improved by diluting the resin to a concentration ofabout 45 per cent solids. Resins prepared as described herein andspray-dried remain stable over periods even longer than one year.

In the production of wet-strengthened paper of the invention thequantity of resin used is within the range ordinarily used in the art ofmaking wet-strengthened paper and varies with the degree of wet strengthdesired. For example, the minimum desirable concentration of resin isthe smallest concentration that gives an appreciable increase in the wetstrength of the paper (i. 6., about 0.1 per cent resin based on theweight of dry pulp). The maximum desirable concentration of resin (i.e., about 10 per cent based on the weight of dry pulp) is that abovewhich the wet strength imparted to paper is not increased enough to makea larger concentration of resin economical for most applications. Aconcentration between about 0.25 and about 5 per cent resin ordinarilygives high wet strength economically and efiiciently. The resin may beadded to the pulp at the wet end of the paper machine as prepared (i.e., as an approximately 50 per cent water solution) or, if desired, itmay be diluted to about 20 per cent solids, preferably with water at atemperature above 50 degrees F. It is desirable to add a catalyst suchas slum or aluminum chloride about five minutes before the resinaddition. The resin and catalyst may stand on the pulp for a few hoursbefore the mixture is used for making paper. Such contact time shouldnot, however, approach as long a period as 24 hours since a portion ofthe wet strength is lost under such conditions. The pH of the mixtureshould be adjusted 9 within a range of 4.0 to 5.5, the pH being loweredto this range using, for example, dilute sulfuric acid. It is mostdesirable that the pH of the mixture be about 4.5

The wet strength of the resin-treated paper of the invention is affectedwhen the pH of the pulp suspension prior to the addition of resin isvaried. The hardness in the water used to dilute the pulp suspension isanother factor which affects wet strength.

The magnitude of the improvement in Wet strength of paper embodying theinvention over previously known wet-strengthened paper may bedemonstrated by tests carried out as follows:

,A resin former (240 grams of urea), methanol-free formalin (713 gramsof a solution consisting of 37 per cent formaldehyde and 63 per centwater) and sodium glycolate containing onehalf molecule of water ofcrystallization (78.5 grams prepared by reacting 82 grams of purified 70per cent hydroxy acetic acid with 28.75 grams of solid sodium hydroxideat a temperature ranging from 70 to 80 degrees C. to a pH of 7.0,filtering the hot mixture and precipitatin th sodium lycolate fromdilute alcohol solution with molecule of water) are mixed together in a1 liter three-necked flask fitted with a thermometer, reflux condenser,stirring rod and oil seals. The DH of the reaction mixture is raised :to7.2 by the addition of sodium hydroxide. The mixture allowed to .standovernight and is then heated on a boiling water .bath to a temperatureof about 95 degrees C. The pH is adjusted to 5.5 bytheadd tion of formicacid, and the solution is held for eight and three-quarters hours at .95degrees C. The resin is allowed to stand overni ht and then held at 60degrees C. for five hours, at a pH of 5.6 before cooling to roomtemperature and neutralizing to a pH of 7.0. This resin is hereinafterreferred to as Resin K.

Anhydrous sodium 'bisulfite (167 grams) is dissolved in methanol-freeformalin (1960 grams of .a solution consisting of 37 per centformaldehyde and 63 per cent water) A resin former (660grams of urea)and sodium acetate (4.5 grams) are dissolved in this solution in a 3liter three-necked flask fitted as described above. The solution isallowed to stand overnight at room temperature. The solution is thenheated on a boiling water bath for about eight and one-half hours atatemperature ranging from 94.5 degrees C. to 97.5 degrees .C. (The bufferaction of the sodium acetate keeps the pH at approximately 5.6 duringthe latter part of this heating period.) The mixture is allowed to standovernight and is then held at a temperature of 60 degrees C. and at a pHof 5.6 for five hours before cooling to room temperature. This resin isused .as a control in the wetstrength tests and is hereinafter referredto as resin B.

A beaten pulp suspension is prepared and treated with samples of resinsK and B by the procedure hereinafter described. The results of wetstrength tests .on sheets made from the resintreated beaten pulpsuspensions are shown in Table 1, in which the headings in columns .2

through 8 indicate the time at which the sample of resin for treatingthe pulp suspension is withdrawn during the resin preparation.

Elllp (containin the equivalent of 360 grams of oven-dried pulp) issoaked in water (10 lite-rs) overnight. The soaked pulp is then agitatedfor 1.0 minutes with pa Lightnin mixer (a high-speed motor-drivenstirrer). The agitated suspension is then placed in a Valley beater (astandard beater designed for laboratory use) and enough water is addedto bring the total volume of water to 23 liters (measured at atemperature of 25 degrees 0.). The beater is run for five minutes (slushperiod) before a load (4500 grams) is placed on the lever arm whichapplies a force to the beater roll. Samp es are with w a ious intervalsduring the beating to measure the rate .at which water passes throughthe pulp (freeness) as Schopper freeness. The freeness of an 800 m1.sample after twenty minutes beating time is about 700 and that for an800 ml. sample after about thirty minutes beating time is 550. Thebeating is terminated after thirty minutes.

The beaten pulp is diluted to such an extent that a volume ofapproximately 300 m1. gives a dry sheet weighing 2.0 grams. The pH isadjusted to 6.5 by the addition of sulfuric acid. Alum (a fresh solutionof 17.75 grams of anhydrous aluminum sulfate in about cc. of water) isadded with stirring to the beaten pulp suspension which is then readyfor the addition of the resin for imparting wet strength. The beatenpulp suspension is allowed to stand for five minutes before a resin forimparting wet strength (an amount of a resin solution suiiicient to give3 per cent based on the weight of the dry pulp) is added. A volume ofstock large enough to give a sheet of the desired 2.0 gram weight 800m1.) is placed in the sheet machine and diluted to a total volume of10.7 liters, and the pH is adjusted to 4.5 by addition of sulfuric acid.The handsheet is made within five minutes after the addition of theresin and the Operation is repeated four times without delay, to makefour more sheets.

The handsheets are made according to Institute of PaperChemistry-Tentative Method 411- B-Valleyf The sheets are pressedseparately between six blotters under a pressure of pounds for twominutes. Each sheet is placed on the drier while still in contact withone blotter (sheet against the metal) and dried for five minutes at 250degrees F. The sheets are conditioned for a 1 minimum of eight hours ina room at a temperature of 75 degrees F., and at 7-8 per cent relativehumidity before testing.

Wet strength measurements are made on the .handsheets with a Mullentester which measures the bursting pressure, expressed as points(approximately pounds per square inch) for a stand.- ardized circulararea. Bursting strength of the paper is given herein as a burst factor,that is, points per 100 pounds of basis weight (basis weight is theweight of 500 sheets of the paper, 25 inches by 40 inches). Mullen wetburst values are obtained on paper samples wet with water from a brush(equivalent to about a ten second soak. of the paper samples).

The great improvement in the wet strength of paper embodying theinvention (paper containing resin K) over previously known syntheticresin impregnated paper (paper containing resin B) is readily apparent.Not only is the maximum Mullen wetburst achieved using resin K muchhigher 11 than that with the control resin, resin B, but it is alsoobtained after a shorter reaction period.

Tests made as follows demonstrate further improvement in the degree ofwet strength imparted to paper in the practice of the present invention.Results of these tests are shown in Tables 2 through 5.

The reaction period for the preparation of a resin such as K may beshortened even more by lowering the pI-Iof the reaction mixture duringthe first stage of the heating (1. e., during the heating at 95 degrees0.), the resulting resin imparting slightly better wet strength than theresin K reacted at a pH of 5.6. The preliminary stage in which the resinis allowed to stand overnight for the formation of dimethylol urea maybe eliminated, and it is not necessary for the resin to stand overnightbefore the heating period at 69 degrees C.

Table 2 shows the results of varying the pH of the resin reactionmixture during the initial heating period at 95 degrees C. Resin K1 isreacted at a pH of 5.45 for six hours at 95 degrees C. (the pH beinglowered by the addition of formic acid) and resin K2 is reacted forthree and one-half hours at a pH of 5.3 (the pH being lowered by theaddition of formic acid). Both resin K1 and resin K2 are then held at 60degrees C. at a pH. of 5.6 in the same manner as resin K, samples beingwithdrawn at various time intervals and used to treat paper prepared andtested as hereinbefore described.

TABLE 2 Mullen Wet Burst-Hours held at 60 degrees C.

The good wet strength obtained by using resin K to treat paper issurpassed by using resin K1 or resin K2, 1. e., using a resin reacted ata pH of around 5.3 or 5.4 instead of 5.6 during the first reactionstage. Maximum Mullen burst is reached when the resins are heated duringthe second stage at 60 degrees C. for about two hours, the decreasingMullen burst after the longer heating periods probably being due to thefact that the resin becomes slightly less soluble upon increasedcondensation.

Considerable development of wet strength occurs during the normal dryingof the paper, due to the rapid rate of cure of resins used in thepresent invention. Complete development of the wet strength can beaccomplished by additional drying of the paper or by storing the resinfor a few days at Warm temperatures. The following table shows theeffect of aging a resin such as K1, the Mullen wet burst being measuredon paper prepared as previously described but dried for only one minuteat 250 degrees F. and conditioned for eight hours at 120 degrees F. atlow humidity. From the table may be seen. also the increase in wetstrength imparted to paper when the catalyst for precipitating the resinis aluminum chloride in place of alum. The last value in columns two andthree is obtained using resin treated paper dried for an additional fourminutes at 250 degrees F. and conditioned for eight hours at 120 degreesF., instead of using the aged resin.

TABLE 3' Mullen Mullen Wet Burst 5 Days Wet Burst (6% alumi- (6% alum)nunicfhlor- 10 5 min. drying of the paper at 250 degrees Table 4 showsthe effect on wet strength of paper embodying the invention when treatedwith resins prepared using various percentages of sodium glycolate(based on the weight of urea) with two different molar ratios offormaldehyde to urea. All resins are prepared by a procedure which isthe same as that described in the preparation of resin K except thatresins (1) and (3) are reacted for eight and threequarters hours at92-965 degrees C., pH 5.6 and for three additional hours at 60 degreesC., pH 5.6; resin (2) for three and one-half hours at 95 degrees C., pH5.3, two hours at 60 degrees C.,

pI-l 5.6; (4) two hours at 95-99 degrees C., pH 5.3, one hour at 60degrees C., pl-I 5.6; (5) two hours at 95-99 degrees C., pH 5.3, twohours at 60 degrees C., pH 5.6; (6) two and three-quarters hours at94-98 degrees C., pH 5.4, one hour at 3 60 degrees C., pI-I 5.6; ('7)two and one-quarter hours at 95-975 degrees C., pH 5.4, one hour at 60degrees C., pH 5.6. The paper is prepared and tested for Mullen burst asdescribed hereinbefore except that the Mullen burst tests for (3) aremade at a temperature of 72 degrees F., 7''! per cent relative humidity.

TABLE 4 Percent Ratio For- 40 Resin Sodium melriehyde Glycolate to UreaIt is evident from Table 4 that the best wet strength using aformaldehyde to urea molar ratio of 2.2 to 1 is obtained by using about30 per cent (based on the weight of urea) of sodium glycolate. The wetstrength is increased by using the more desirable formaldehyde to ureamolar ratio of 2.12 to 1.

As hereinbefore mentioned, use of excessive amounts of a modifying agentsuch as sodium glycolate in a urea-formaldehyde resin tends to decreasethe degree of wet strength imparted to paper by the resin. The Mullenwet burst value obtained using 34.8 per cent of sodium glycolate isessentialy no higher than that obtained by using the resin prepared with25.19 per cent of sodium glycolate. Similarly, al-

though a resin prepared with a formaldehyde to urea molar ratio of 2.12to 1 using 52.5 per cent of sodium glycolate imparts Mullen burst ashigh as 3%.2, greater wet strength can be obtained with a much smallerpercentage of sodium glycolate such as 33.5 per cent or 31.4 per cent.

Table 5 shows the efiect of varying the concentration of alum and resinin the preparation of wet strengthened paper. The paper treated isprepared and tested by the procedure hereinstis before described, the pHindicated being that of the pulp adjusted with sulfuric acid before theaddition of alum and resin. It is seen from the table that it is mostdesirable to use about 3 per cent of alum and 3 per cent of resin (basedon the weight of dry pulp) when the pulp is at a pH of about 6.5 to 7.0.(The resin is one prepared by a procedure similar to that described forresin K).

TABLE Mullen Percent Percent Mullen Burst Alum Rosin f f (pH 6.5-7.0)

5 5 8. 5 8. 5 1 5 9. 5 9. 2 3 5 10.1 9. 4 6 .5 9.6 8. 5 5 3 l3. 2 l3. 9l 3 l8. 5 22. 0 3 3 25. 8 27. 7 6 3 25. 7 25. 2

In the practice of the invention resins prepared using urea as the resinformer are preterred to resins in which melamine is used not onlybecause urea resins impart higher wet strength to paper but also becausethey are cheaper to use than melamine resins. -t may be desirable,however, in some cases to replace some or all of the urea with melaminein order to obtain certain advantages characteristic of melamine resins.For example, sodium glycolate melamine resin is superior to sodiumglycolate urea resin when it is desired to obtain wetstrengthened paperhaving high humidity endurance. This may be demonstrated by testscarried out as follows:

Glycolic acid (763 grams in 32.7 cc. of water) is mixed with flakecaustic (45 grams). Methanol-free formalin (242 grams of a solutionconsisting of 37 per cent formaldehyde and 63 per cent Water) is heatedwith this mixture in a 3 liter three-necked flask fitted with a ther--mometer, reflux condenser, stirring rod and oil seals, over a boilingwater bath for two hours at a temperature of 95 degrees C. The pH of themixture is 6.0. At the end of this heating period melamine (63 grams)and sodium bicarbonate 1.68 grams) are added to the mixture, which isthen held for twenty minutes at a temperature of 7.0 degrees C. The pHof the mixture is 7.5. Additional melamine (63 grams) and methanolfreeformalin (283 grams of a solution consisting of 37 per cent formaldehydeand as per cent water) are then added, and the mixture is held for oneand three-quarters hours at temperatures ranging from 90 to 95 degreesC.

A sodium glycolate-urea resin in which ten per cent of the urea byweight is replaced with melamine is prepared by the procedure describedfor resin K2, the melamine being added after about two hours of theheating at 95 degrees C.

Samples of these two resins and samples of resin K (prepared ashereinbefore described) are used to treat paper prepared and tested forMullen wet burst as hereinbefore described and for wet tensile strength.Other resin-treated paper samples are conditioned for ten and twentydays in an oven at a temperature of 120 degrees F., and at 75 per centrelative humidity, and are tested for wet tensile strength. The resultsof these tests are shown in Table 6.' Wet tensile measurements, made ona standard pendulumtype tensile tester, are given in grams per mm. paperstrip, tested after soaking for one hour in water at 23 degrees C.

TABLE 6 Days at 120 F. and 75 percent R. 11.

Percent of Resin Mullen Original Burst Wet Tensile Strength Wet iilen-Strength 0 10 20 After 20 Resin K (100% urea) 29. 6 l, 270 430 320 2590% Urea, 10% Melamin 28. l l, 200 590 355 30 100% Melamine .l 21. 3 915495 360 39 As indicated by the results in Table 6, wetstrengthened papercontaining a sodium glycolate-melamine resin has improved humidityresistance over wet-strengthened paper containing a sodiumglycolate-urea resin, although the wet tensile strength of the former isinitially poorer than the latter.

In summary, paper of the instant invention possesses greater wetstrength than the resintreated paper heretofore known, because the resinused to treat paper of the invention is modified with an electrolyticgroup which imparts a high degree of water solubility to the resin. Thehydroxy or amino group in the substituted carboxylic acid modifyingagent from which the electrolytic group is derived condenses with amethylol group in the resin molecule so that the electrolytic group canbecome part of the resin molecule. Because of the high reactivity of anamino group with a methylol group, it would be expected that thereaction whereby the electrolytic group becomes part of the resinmolecule would be facilitated and a greater improvement inwet-strengthening properties would result when an amino-substitutedearboxylic acid is used as the modifying agent rather than ahydroxy-substituted carboxylic acid. However, it has been found on thecontrary that better results are obtained in the instant invention whenthe carboxylic acid used is hydroxy-substituted. The specific reason whya hydroxy-suhstituted carboxylic acid gives better results in theinstant invention is not known, but it is believed that there may be atendency for free formaldehyde present to react with the amino group inan amino-substituted carboxylic acid, and that such a separate reactionmay interfere with the reaction that is desired in the practice of theinstant invention.

The following examples illustrate the practice of the invention:

Example 1 A substituted carboxylic acid (252 grams of glycolic acid in108 cc. of water) is mixed with a caustic solution (100.3 grams of flakecaustic in 131.7 cc. of water) and maintained at temperaturesrangingrfrom 7-0 to degrees C. for 25 minutes. To this mixture in a 3liter threenecked flask fitted with a thermometer, reflux condenser,stirring rod and oil seals is added a resin former (690 grams of urea)and methanolfree formalin (1980 grams of a solution consisting of 3'7per cent formaldehyde and 63 per cent water). The pH of the mixture isadjusted within the range 6.5 to 7.0 with additional iiake caustic, andthe mixture is heated to a tenperature of about degrees C. The pH isthen adjusted to 5.4 with glycolic acid and the heating is continued atapproximately 95 degrees C. for two and one-half hours. The mixture isthen cooled to 60 degrees C., (the pH being adjusted to 5.6 with sodiumhydroxide during the cooling) and is held at this temperature for onehour. The resin is then cooled to room temperature, and the pH isadjusted to 7.0 with sodium hydroxide.

A beaten pulp suspension of any type of paper pulp, such as bleached orunbleached sulphite, kraft, or ground wood pulp, is prepared, inaccordance with the procedure hereinbefore described, for the additionof a resin for imparting wet strength. A resin for imparting wetstrength (the resin prepared as described in the preceding paragraph inan amount sufiicient to give 3 per cent based on the weight of dry pulp)is added to the beaten pulp suspension. Handsheets made as hereinbeforedescribed from such a resin-treated beaten pulp suspension are superiorin wet strength to resin-impregnated paper sheets heretofore known.

Example 2 Paper having superior. wet strength is prepared as describedin Example 1 except that the resin solution added to the beaten pulpsuspension is prepared by one of the following procedures:

(a) To a substituted carboxylic acid (85 grams of lactic acid in 15 cc.of Water) in a 1 liter threenecked flask fitted with a thermometer,reflux condenser, stirring rod and oil seals, is added flake caustic (38grams), and the pH of the mixture is adjusted to 6.8 by the addition ofdilute sodium hydroxide. A resin former (230 grams of urea) andmethanol-free formalin (660 grams of a solution consisting of 37 percent formaldehyde and 63 per cent water) are then added to the flask andthe mixture is held for three hours at a temperature of about 95 degreesC., the pH being adjusted to approximately 5.5-5.6 with lactic acid whenthe temperature reaches 89 degrees C. and lowered to 5.4 with additionallactic acid during the last fifteen minutes of the heating. The mixtureis then cooled to 60 degrees C. The pH is raised to 5.6 with dilutesodium hydroxide, and the solution is held at 60 degrees C. for onehour. The resin is cooled to room temperature and neutralized withdilute sodium hydroxide to a pH of 7.0.

(b) A substituted carboxylic acid (75 grams of tartaric acid in 10 cc.of water) is mixed with flake caustic grams), methanol-free formalin(344 grams of a solution consisting of 37 per cent formaldehyde and 63per cent water) and a resin former (120 grams of urea) by the proceduredescribed in (a). The mixture is held for four hours at a temperature of95 degrees C., the pH being adjusted to 5.4 with tartaric acid when thetemperature reaches 90 degrees C. The mixture is then cooled to 60degrees C. and held at this temperature for one hour, the pH beingadjusted to 5.6 with sodium hydroxide at the beginnin of this stage ofthe reaction. The resin is then cooled to room temperature andneutralized with dilute sodium hydroxide to a pH of 7.0.

The procedure described in the preceding paragraph may be carried outusing, as the substituted carboxylic acid, glutamic acid (75 grams) orglycine (38 grams).

(0) Glycoiic acid (105 grams in cc. of water), flake caustic (52.9grams), methanol-free formalin (569 grams of a solution consisting of 37per cent formaldehyde and 63 per cent water) paraformaldehyde (44grams), and urea (240 grams) are mixed and heated as described in (a).When the temperature approaches 90 degrees C. the pH is adjusted to 5.4with glycolic acid. The heating is continued for one and one-half hoursat temperatures ranging from 95 to 100 degrees C. The mixture is cooledto degrees C. and held at this temperature for one-half hour, the pHbeing raised to 5.6 at the beginning of this period with sodiumhydroxide solution. The resin is then cooled to room temperature andneutralized with dilute sodium hydroxide to a pH of 7.0.

(d) A resin former (720 grams of urea) and methanol-free formalin (2064grams of a solution consisting of 37 per cent formaldehyde and 63 percent water) are mixed in a three liter three-necked flask fitted with athermometer, reflux condenser, stirring rod and oil seals. The pH of themixture is adjusted with sodium hydroxide within the range 6.5 to 7.0.Crystalline H20 (370 grams) is added and the mixture is heated to atemperature of 95 degrees C. The pH is adjusted to approximately 5. Lwith glycolic acid and the mixture is reacted at this temperature and pHto a viscosity of 48 seconds (Ford cup viscosity after about two andthree-quarters hours heating). The mixture is then cooled to atemperature of 60 degrees C., the pH is adjusted to 5.6 with sodiumhydroxide, and the reaction is continued until the viscosity of thesolution is 66.5 seconds (Ford cup viscosity after about one andone-half hours heating at 60 degrees C.). The resin is cooled to roomtemperature and the pH is adjusted to 7.0 with sodium hydroxide.

(e) A substituted carboxylic acid (215 grams of an aqueous solutionconsisting of per cent glycolic acid and 30 per cent water) is mixedwith flake sodium hydroxide (85 grams) in a 2 liter three-necked flaskfitted with a thermometer, reflux condenser, stirring rod and oil seals,

and the mixture is maintained with cooling for about eight minutes attemperatures from to degrees C. Methanol-free formalin (874 grams of anaqueous solution consisting of 50 per cent formaledhyde and 50 per centwater, prepared by vacuum distillation of commercial 37 per centformalin) is added, and the pI-I of the mixture is adjusted within therange 6.5 to 7.0 by addition of sodium hydroxide. A resin former (413grams of urea) is dissolved in this mixture, the pH being adjusted to7.5 when the temperature, which drops sharply and then starts risingslowly, reaches 50 degrees C. (held with cooling during the adjustment).The temperature of the mixture is raised to degrees C. by heating over aperiod of about forty minutes, the pH being adjusted to 7.2 with sodiumhydroxide when the temperature reaches 75 degrees C. The heating iscontinued. at 95 degrees C. for five minutes before the pH is lowered to5.6 with glycolic acid. The solution is reacted to a viscosity of K-L(Gardner-Holdt) which is attained about one and one-quarter hours afterthe mixture reaches 95 degrees C. The mixture is cooled to 60 degrees C.(over a period of about ten minutes) and held at this temperature and ata pH of 5.6 until the viscosity of the resin is V-W (approximately twoand one-quarter hours of heating at 60 degrees 0.). The resin is cooledto room temperature, the pH is adjusted to 8.0 with sodium hydroxide,and water (167 grams), and an aqueous bufier solution (12.7 grams of amixture comprising 65 per cent boric acid and 35 per cent borax in 114.3cc. of water) are added.

What is claimed is:

1. Paper of improved wet strength containing 0.1-10 per cent of its drypulp weight of a condensation product of (w) formaldehyde (b) asubstance of the class consisting of urea and melamine and (c) 0.05-0.25mol per mol of (b) of a carboxylic acid derivative of the classconsisting of substituted aliphatic carboxylicacids and alkali metalsalts thereof, whose molecule has not more than eight carbon atoms andhas from two to four carbon atoms per carboxy group; said condensationproduct containing no monovalent substituents derived from (w), (b) andother than NH2, OH, -CO'OH and COOM groups, wherein M is an alkalimetal, the molar ratio of (a) to (1)) being from 1.8:1 to 22:1 when (b)is urea and the molar ratio of (a) to (1)) being from 4:1 to 7:1 when(b) is melamine.

2. Paper of improved wet strength containing 0.1-10 per cent of its drypulp weight of a condensation product of (a) 1.8-2.2 mols offormaldehyde (b) 1 mol of urea and (c) 0.15-0.25 mol of a carboxylicacid derivative of the class consisting of substituted aliphaticcarboxylic acids and alkali metal salts thereof, whose molecule has notmore than eight carbon atoms and has from two to four carbon atoms percarboxy group; said condensation product containing no monovalentsubstituents derived from (a), (b) and (0) other than NH2, -OH, COOI-Iand COOM groups, wherein M is an alkali metal.

3. Paper of improved Wet strength containing a condensation product asclaimed in claim 2 wherein carboxy groups are neutralized with an alkalimetal base.

4. Paper of improved wet strength as claimed in claim 3 wherein thecarboxylic acid derivative has twocarbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,922,690 Lougovoy Aug. 15, 1933 2,322,887 I Schwartz et a1June 29, 1943 2,325,302 Britt July 27, 1943 2,338,602 Schur Jan. 4, 19442,389,415 DAlelio Nov. 20, 1945 2,443,368 Alexander et al. June 15, 19482,446,991 Alexander et al Aug. 17, 1948 2,524,111 La Piana et a1 Oct. 3,1950 2,524,112 La Piana et al. Oct. 3, 1950 2,601,666 Niles June 24,1952

1. PAPER OF IMPROVED WET STRENGTH CONTAINING 0.1-10 PER CENT OF ITS DRYPULP WEIGHT OF CONDENSATION PRODUCT OF (A) FORMALDEHYDE (B) A SUBSTANCEOF THE CLASS CONSISTING OF UREA AND MELAMINE AND (C) 0.05-0.25 MOL PERMOL OF (B) OF A CARBOXYLIC ACID DERIVATIVE OF THE CLASS CONSISTING OFSUBSTITUTED ALIPHATIC CARBOXYLIC ACIDS AND ALKALI METAL SALTS THEREOF,WHOSE MOLECULE HAS NOT MORE THAN EIGHT CARBON ATOMS AND HAS FROM TWO TOFOUR CARBON ATOMS PER CARBOXY GROUP; SAID CONDENSATION PRODUCTCONTAINING NO MONOVALENT SUBSTITUTENTS DERIVED FROM (A), (B) AND (C)OTHER THAN -NH2, -OH, -COOH AND COOM GROUPS, WHEREIN M IS AN ALKALIMETAL, THE MOLAR RATIO OF (A) TO (B) BEING FROM 1.8:1 TO 2.2:1 WHEN (B)IS UREA AND THE MOLAR RATIO OF (A) TO (B) BEING FROM 4:1 TO 7:1 WHEN (B)IS MELAMINE.